Disorders of sex development (DSD) encompass a wide range of urogenital anomalies, ranging from mild hypospadias to sex reversal with genital ambiguity. Impressively, these disorders occur in approximately 0.5-1% of live births. Despite the prevalence of these disorders, the molecular mechanisms behind the transformation of the bipotential gonad into a testis or an ovary are not completely understood. Genetic studies of 46, XX or 46, XY sex reversal patients determined that mutations in either SRY or Steoidogenic Factor-1 (SF1) account for 30% of all isolated 46, XY Gonadal Dysgenesis (GD) where SRY is present by FISH analysis. Other rare and isolated causes of 46, XY GD include mutations in Chromobox-2 (CBX2), deletions in 9p or 10q, and duplications of SOX9. However, these genes account for less than 1% of all cases. This implies that there exist many undiscovered genes that are important in the developing mammalian gonad. In this proposal, we seek to better understand the molecular mechanisms behind mammalian sex determination using a powerful mouse model of C57BL/6J-YPOS sex reversal. The presence of the Mus domesticus poschiavinus Y chromosome, YPOS on the C57BL/6J (B6) genetic background causes 60% of B6- YPOS animals to be sex reversed, developing as adult females (2 ovaries) or hermaphrodites (1 ovary, 1 testis). B6-YPOS sex reversal occurs early in embryonic development, as all E15.5 animals develop some degree of ovotestis. We have developed a congenic strain in which a novel locus of 129-origin on chromosome 11 confers significant protection from B6-YPOS sex reversal. In this proposal, we will narrow the size of the congenic region on chromosome 11 to a manageable size for further study. To date, we have created 10 subcongenic lines, each containing a subset of the original congenic line, and preliminary studies in adult XY animals have identified a subcongenic line in which a small region of 1.3 Mb is sufficient to confer the protection phenotype. Using bioinformatics and mRNA and protein expression studies, we will investigate a small number of candidate genes in our 1.3 Mb region. The function of the top three candidate genes in sex determination will be assayed using gonadal organ culture. The third aim will focus on a complementary and independent approach to investigate the functional role of the congenic region, rather than a specific gene, to understand the mechanism behind the congenic regions'protective effect in early gonadal development. What we gain from this newfound understanding of sex determination will ultimately allow for better diagnosis and management of patients with disorders of sex development. PUBLIC HEALTH RELEVANCE: In this proposal, we investigate the molecular and cellular mechanisms that define the decision of the bipotential gonad to become a testis or an ovary;a process which is triggered by the expression of a male- specific gene, Sry, on the Y-chromosome. As only 25% of human cases of XY-sex reversal are explained genetically, we propose to identify new genes involved in this process using a mouse model of XY-sex reversal. This proposal will elucidate basic questions about how males and females diverge, beginning at gonadal development in utero, and will improve genetic classification and diagnostic methods of patients born with disorders of sex development.